Sr latch circuit with single gate delay

ABSTRACT

An SR latch circuit with single gate delay is provided. The circuit has an an enable input and an SR latch. There is first input stage having an input for receiving a set input and having an output for producing a first component of the SR latch circuit output, the first input stage having only one transistor that receives the enable input, the first input stage becoming transparent while enabled, the first input stage having a single gate delay between the input of the first input stage and the output of the first input stage. There is a second input stage having an input for receiving a reset input and having an output for producing a second component of the SR latch circuit output, the second input stage having only one transistor that receives the enable input, the second input stage becoming transparent while enabled, the second input stage having a single gate delay between the input of the second input stage and the output of the second input stage.

FIELD

The application relates to SR latch circuits.

BACKGROUND

An SR latch circuit (Set/Reset) is an asynchronous device: it worksindependently of control signals and relies only on the state of the Sand R inputs. When a high is applied to the Set line of an SR latch, theQ output goes high. The SR latch circuit has a feedback mechanism thatcauses the Q output to remain high, even when the S input goes lowagain. This is how the latch circuit serves as a memory device.Conversely, a high input on the Reset line will drive the Q output low,effectively resetting the latch's “memory”. When both inputs are low,the latch circuit “latches”—it remains in its previously set or resetstate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theattached drawings in which:

FIG. 1 is a simplified block diagram of an SR latch circuit with singlegate delay provided by an embodiment of the invention;

FIG. 2 is a simplified circuit diagram of an SR latch circuit withsingle gate delay provided by an embodiment of the invention;

FIGS. 3 and 4 are timing diagrams associated with FIG. 2;

FIG. 5 is a simplified circuit diagram of another SR latch circuit withsingle gate delay provided by an embodiment of the invention;

FIGS. 6A and 6B are simplified circuit diagrams for alternatives for astorage circuit;

FIG. 7 is a simplified block diagram of a decision feedback equalizer;

FIG. 8A is a simplified circuit diagram of a Strong-Arm slicer and an SRlatch, provided by an embodiment of the invention;

FIG. 8B is a timing diagram associated with FIG. 8A;

FIG. 9 is a simplified block diagram of a SerDes receiver;

FIG. 10 is a simplified block diagram of a single tap unrolled DFE(decision feedback equalizer);

FIG. 11 is a simplified block diagram of a system employing 25G NRZtechnology.

DETAILED DESCRIPTION

Conventional SR latches can add significant delay in a high-speed datapath as they are typically composed of multiple stages (with multiplegate delays). Conventional SR latches typically have at least a two gatedelay between input and output. This can be problematic for use incircuits requiring high speed, for example in a decision feedbackequalizer used in high speed applications. In particular, for an SRlatch circuit added in a feedback path, speed can be critical to overallsystem performance.

According to one aspect of the present invention, there is provided anSR latch circuit having an enable input and an SR latch circuit outputcomprising: a first input stage having an input for receiving a setinput and having an output for producing a first component of the SRlatch circuit output, the first input stage having only one transistorthat receives the enable input, the first input stage becomingtransparent while enabled, the first input stage having a single gatedelay between the input of the first input stage and the output of thefirst input stage; second input stage having an input for receiving areset input and having an output for producing a second component of theSR latch circuit output, the second input stage having only onetransistor that receives the enable input, the second input stagebecoming transparent while enabled, the second input stage having asingle gate delay between the input of the second input stage and theoutput of the second input stage.

According to another aspect of the present invention, there is provideda method comprising: controlling a first input stage and a second inputstage to be transparent with an enable input; receiving a set input atthe first input stage and receiving a reset input at the second inputstage, the set and reset input each being a RZ (return to zero) signalthat is timed to become valid after the enable input has caused theinput stages to become transparent; the first input stage generating afirst component of a complementary SR (set reset) latch circuit outputwith a single gate delay between the set input and the first componentof the complementary SR latch circuit output; the second input stagegenerating a second component of a complementary SR (set reset) latchcircuit output with a single gate delay between the set input and thesecond component of the complementary SR latch circuit output.

Referring to FIG. 1, shown is an SR latch circuit provided by anembodiment of the invention. The SR latch circuit has a first inputstage 50 having an input for receiving a set input and having an outputfor producing a first component op of the SR latch circuit output. Thefirst input stage has only one transistor 51 that receives an enableinput. The first input stage becomes transparent while enabled, and hasa single gate delay between the input of the first input stage and theoutput of the first input stage. In the illustrated example, the enableinput is enb (enable bar), such that the input stage becomes transparentwhile enb is low. Specific examples of such an input stage are describedbelow. Also shown is a second input stage 52 having an input forreceiving a reset input and having an output for producing a secondcomponent on of the SR latch circuit output. The second input stage hasonly one transistor 53 that receives the enable input. The second inputstage becomes transparent while enabled, and has a single gate delaybetween the input of the second input stage and the output of the secondinput stage.

The transistors 51,53 are PMOS and NMOS transistors respectively.However this is implementation specific.

The description above refers to single gate delay. Conventionally,single gate delay is defined to exist when the longest path from inputdata to output data is the delay time of a single transistor gateturning on. However, note that the delay of a single gate can vary. Ingeneral the larger the device the lower the delay, but the higher thepower. However, there is a lower limit to the delay regardless of sizethat has to do with the time for a transistor channel to turn on.

In some embodiments, the latch circuit is configured to receive set andreset input signals as RZ (return to zero) signals that are timed tobecome valid after the enable input has caused the input stages 50,52 tobecome transparent. These may, for example, be received from an inputgenerating circuit that generates the set input and the reset inputsignals. The input generating circuit may, for example, be a strong-armslicer circuit.

Also shown is a storage circuit 62 having first and second inputscross-connected with first and second outputs, the first output alsoconnected to the output of the first input stage 50, and the secondoutput also connected to the output of the second input stage 52. Thestorage circuit functions to hold the values at the outputs of the inputstages. However, for high frequency operation, the storage circuit canbe omitted in which case the circuit can rely on parasitic capacitanceat the output to hold the values for a long enough period.

In some embodiments, the latch circuit is configured for return-to-zerosignaling for set and reset inputs, and for non-return-to-zero signalingfor the SR latch circuit output. In some embodiments, the input stageshave a transparent state activated by the enable, and the input stagesare configured so as to overpower the storage circuit while in thetransparent state.

Referring to FIG. 2, shown is an SR latch circuit provided by anembodiment of the invention. This is a specific example of the SR latchcircuit of FIG. 1. The circuit has set and reset nodes 140,142 forreceiving set and reset input signals, and an enb node 107 for receivingan enable input enb, enb being short form for enable bar, or thecomplement of an enable signal. The output of the circuit is taken atnodes on, and op.

The set node 140 is connected to inputs of a PMOS transistor 102 and anNMOS transistor 104 which together form an inverter circuit 100.Transistor 102 is also connected to PMOS transistor 106 which isconnected to enb node 107. More specifically, set node 140 is connectedto the gates of transistors 102,104. The source of transistor 104 isconnected to ground. The drains of transistors 104,102 are connectedtogether and to node on. The source of transistor 102 is connected tothe drain of transistor 106. Enable input enb is received at the gate oftransistor 106. The source of transistor 106 is connected to power.

The inverter circuit 100 in combination with PMOS transistor 106constitute a specific example of the first input stage 50 with singlegate delay of FIG. 1. Note that the enable input enb is received at asingle transistor 106.

Similarly, the reset node 142 is connected to inputs of a PMOStransistor 132 and an NMOS transistor 134 which together form aninverter circuit 130. Transistor 132 is also connected to PMOStransistor 136. PMOS transistor 136 is connected to enb node 107.

More specifically, reset input 142 is connected to the gates oftransistors 132,134. The source of transistor 134 is connected toground. The drains of transistors 134,132 are connected together and tonode op. The source of transistor 132 is connected to the drain oftransistor 136. Enable input enb is received at the gate of transistor136. The source of transistor 136 is connected to power.

The inverter 130 in combination with PMOS transistor 136 form a specificexample of a second input stage 52 with single gate delay circuit ofFIG. 1. Note that the enable input enb is received at a singletransistor 132.

Also shown is a storage circuit 110 composed of cross-coupled inverters112,120. Inverter 112 is composed of PMOS transistor 114 and NMOStransistor 116, and has an input connected to node op and an outputconnected to node on. Inverter 120 is composed of PMOS transistor 122and NMOS transistor 124, and has an input connected to node on and anoutput connected to node op.

In the description of FIG. 2, the power supplies have been referred toas power and ground. More generally, any appropriate power supplies forCMOS circuitry may be employed. Other example naming conventions includepositive and negative, or vdd and vss.

Storage circuit 110 is a specific example of the storage circuit 62 ofFIG. 1.

With the circuit of FIG. 2, the enable is active low, and signals atnodes set and reset are active high. The set and reset signals carrycomplementary input data, and use RZ (return to zero) signaling. Withcomplementary input data, (set,reset)=(1,0) for a one, and(set,reset)=(0,1) for a zero. Data is valid for half the enable cycle,and then returns to zero for the other half-cycle. The output signals atthe on and op nodes are complementary NRZ (non-return-to-zero) signals,and are valid for the whole enable cycle. (op,on)=(1,0) for a one, and(op,on)=(0,1) for a zero.

An operating objective is that only one of set and reset is allowed tobe high at a time and furthermore, this is only allowed when enb is low.This objective may be achieved through appropriate definition of theenb, set and reset signals in preceding circuitry. Enb is related to aclock used to generate set and reset. Making enb earlier than set andreset can increase the speed, but at the cost of higher power. This willbe detailed further below in the discussion of the timing diagrams.

With the circuit of FIG. 2, when enb is low, the inverter circuits100,130 behave like regular inverters, sensing new data. During thistime, the inverter circuits 100,130 overpower the storage circuit 110.This can be achieved, for example, by using larger transistors for theinverter circuits 100,130 than for the storage circuit 110.

When enb is high, the inverter can pull low, but cannot pull high. Inother words, a “one” input will be pulled low, but a “zero” input willnot be pulled high. During this time, the latch circuit is holding ontoprevious data. However, as detailed below, only a (1,0) input is validwhen enb is high.

It is noted that the inverter circuit 100 in combination with PMOStransistor 106 differs from a conventional clocked inverter. Aconventional clocked inverter would include a clocked NMOS transistorconnected to transistor 104. The two clocked transistors would receive aclock input clock, clockb that in effect gates the input data. Thisadditional transistor would pull a zero high when clockb is high.However, in addition, this transistor increases the input capacitance ofthe circuit in that now, both the additional transistor and transistor104 need to be charged to generate a one on output node on. With such aconventional clocked inverter circuit, there would be a one gate delay.However, if the clocked NMOS was sized the same as the input NMOS, thedelay may be twice as large. Another way to look at it is to achieve thesame delay as the provided circuit the input NMOS and clocked NMOS mayneed to be twice as large (and therefore increasing loading on previouscircuit). A reduced input capacitance can help with the performance ofpreceding stages. In contrast, to a clocked inverter, where a clockgates the data, in some embodiments an enabled signal is used to enablethe input stage in advance of the input data going valid. In this sense,the enable signal is not gating the input data.

FIG. 3 is a timing diagram illustrating the latching of a “1”, where theprevious value had been a “0”. Recall, a “1” input is represented by(1,0) at input nodes set, reset. The timing diagram shows plots for enb,set, reset, op and on. The starting state is enb high, RZ signals(set,reset)=(0,0), and NRZ signals (op,on)=(0,1) representing a “0”which is the previous latch circuit state.

On the reset side of the circuit, the enb goes low at 200 causes theinverter 130 to function as a normal inverter. The reset input is zeroand enb low, so this pulls op high at 202 which involves chargingtransistor 134.

On the set side of the circuit, enb going low causes inverter circuit100 to function as a normal inverter. When set goes high at 204, thispulls on low at 206.

FIG. 4 is a timing diagram illustrating the latching of a “1”, where theprevious value had been a “1”. As before, a “1” input is represented by(1,0) at input nodes set, reset. The timing diagram shows plots for enb,set, reset, on and op. The starting state is enb high, RZ signals(set,reset)=(0,0), and NRZ signals (op,on)=(1,0) representing a “1”which is the previous latch circuit state.

On the reset side of the circuit, the enb goes low at 210 causes theinverter circuit 130 to function as a normal inverter. The reset inputis low and enb is low which are consistent with op being high, so thereis no change at node op.

On the set side of the circuit, enb going low at 210 causes invertercircuit 100 to function as a normal inverter. While set is still low,node on starts transitioning towards an on state at 214, but when setgoes high at 212, this pulls on back to low at 216.

Note that the behavior of the circuit to latch circuit a ‘0’ is the sameas described for FIGS. 3 and 4, but with the signaling for op and onreversed.

Advantageously, it can be seen with the example of FIG. 4, on begins tocharge towards a one state, triggered by enb going low, irrespective ofwhether it is set or reset that ultimately goes high. If it is set thatgoes high, on is pulled back to zero. However, if it is reset that goeshigh, on would continue to be pulled high (as in the example of op beingpulled high for set going high in the example of FIG. 3). The earlierthat enb is relative to set/reset, the more that on/op is charged. Thiscan increase speed in that on/op reach their fully charged state sooner,relative to set/reset, than they would but for the precharge triggeredby enb going low. However, there is a trade off with power consumption.Extra power is consumed in order to partially charge on/op even in caseswhere they are not ultimately going to be pulled high.

The enb, which is based on a clock used to generate SR latch circuitinput pulses (set/reset) in the previous stage is also used topre-emptively begin the switching of the SR latch circuit output, beforeinput data is available. Before the output fully switches, the inputdata then determines whether the output should fully switch or not.

It can be seen that with the provided circuit, there is a single gatedelay between the set/reset input and the op/on output. For example,when set goes high, on goes low after a single gate delay introduced byinverter 100. The output of inverter 100 appears at node on immediatelyafter processing by the inverter, notwithstanding the presence of thestorage circuit 110. Similarly, when reset goes high, op goes low aftera single gate delay introduced by inverter 130. The output of inverter130 appears at node op immediately after processing by the inverter,notwithstanding the presence of the storage circuit 110.

Table 1 below is a truth table for the operation of the circuit of FIG.2. In the truth table, “p” means the previous state, and “x” meansunknown.

TABLE 1 Enb Set Reset on op Valid states 0 0 1 1 0 0 1 0 0 1 1 0 0 p pInvalid states 1 0 1 1 0 1 1 0 0 1 1 1 1 x x Bit Error 0 0 0 1 1 0 1 1 00

The valid states include enb=0, in combination with one and only one ofset and reset high. Another valid state is enb high with set and resetlow. In this state the circuit is in a latched, non-transparent state.However, it can be seen that if either of set or reset goes high whileenb is high, this is reproduced at the output. In this sense, thesestates are invalid.

This behavior can be contrasted with a conventional clocked SR latchcircuit the truth table for which is reproduced in Table 2.

TABLE 2 clk Set Reset on op 0 0 1 1 0 0 1 0 0 1 1 0 0 p p 1 0 1 p p 1 10 p p 1 1 1 p p

The conventional clocked SR latch circuit also has bit error states when(clk, set, reset)=(0,0,0) or (0,1,1). The behavior in these states isimplementation dependent.

It can be seen that the output when clk is high is valid for any set andreset inputs. However, the cost of this is increased delay, possibly afactor of two, assuming the same size NMOS transistors.

FIG. 5 is a circuit diagram of another clocked SR latch circuit providedby an embodiment of the invention. This circuit is similar to that ofFIG. 2, but whereas the circuit of FIG. 2 is based on an enable that isactive low and set, reset active high, the circuit of FIG. 5 is based onan enable that is active high, and set, reset active low.

The valid states can be summarized as follows:

En Setb Resetb on op 1 0 1 0 1 1 1 0 1 0 0 1 1 p p

The provided circuit may save data path and clock path power for thesame delay, at the cost of introducing invalid states. However, thesestates are not a problem so long as the previous stage generates validinput signals.

The circuit of FIG. 2 includes a storage circuit 110 implemented using apair of inverter implemented as shown. It should be understood this is aspecific example. Other circuits with cross-coupled inputs and outputscan be employed.

In some embodiments, where very fast clock/data rates are employed, thestorage circuit is omitted. Such a circuit would rely on the parasiticcapacitance to hold the output long enough before the next clock cyclebegins. Parasitic capacitance is the capacitance from existing devicesin the circuit and wiring, without an explicit storage circuit.

Another storage circuit example is depicted in FIG. 6A, where capacitorsat nodes op and on are used as the storage cells. As mentioned above, atvery high data rates parasitic capacitance could be used.

FIG. 6B is a circuit diagram of another example of a storage circuitfeaturing cross-coupled clocked inverters. Note that this also be donewith only enable PMOS or only enabled NMOS as well. For such a storagecircuit, the enable phase in this case would have to be opposite to theinput stages. That is to say, when the input stages are transparent,these enabled devices are off

It is noted that a conventional clocked SR latch circuit would generallybe used to retime data to the clock. In the described embodiment, theenable is adjusted to be synchronized to the data (or vice-versa), sothat there is not any retiming, thereby reducing delay.

Compared to a conventional SR latch, there are two improvements that maybe realized in some instances:

-   -   1) by using a SR latch circuit with an enable that is        synchronous to the data, this makes the latch circuit        transparent whenever data is valid, thereby achieving a single        gate delay. This may also reduce the load seen by the previous        stage compared to a conventional SR latch.    -   2) removing a clocked device from a conventional clocked SR        latch, thereby further reducing the load on the previous stage        (for the same delay).    -   3) reducing clock path loading, thereby reducing clock path        power.

FIG. 7 is a simplified block diagram of an example decision feedbackequalizer (DFE) featuring a clocked SR latch circuit provided by anembodiment of the invention. The DFE has a shift register 900 that ismade up of delay cells having “T” delay, where T is the baud period(baud period is the duration of a bit). The example depicts three delayscells, but that is implementation specific. Outputs of the delay cellsare weighted in gain stage 902 with gains g0,g1,g2 and combined with theinput data at 904. The combined output is passed through slicer 906 theoutput of which is the recovered data. This output is also fed back tothe shift register.

The provided SR latch circuit can be used as a building block toimplement this shift register. The provided SR latch circuit can be usedin sub-rate or full-rate applications. It can be used wherever aconversion from return-to-zero to non-return-to zero signaling isneeded. For example, in FIG. 7, the signals to the g0/g1/g2 gain stagesmust be NRZ.

FIG. 8A depicts an example of a previous stage (a Strong-Arm slicer),generally indicated at 950, for driving the provided SR latch, anexample of which is depicted at 952. A timing diagram for the combinedfunctionality of the Strong-Arm slicer 950 and the SR latch circuit 952is depicted at FIG. 8B. With this example:

-   -   the enb signal is active low. The Strong-Arm slicer is activated        by the falling edges (see the arrows on the falling enb        waveform);    -   the Strong-Arm slicer is edge activated: when enb falls the        input data is sensed and amplified at the output; when enb rises        the outputs are pre-charged (pulled to ground);    -   although large ip/in signals are depicted for the example, a        typical Strong-Arm slicer is designed to be able to sense very        small ip/in amplitudes (as small as a few mV in some cases) and        output large amplitudes (reaching power supply amplitudes);    -   the example shows three samples taken by the strong-arm slicer,        and how the SR latch circuit transitions:    -   1. Transition from 0 to 1 (set goes high, op/on=10)    -   2. Transition from 1 to 0 (reset goes high, op/on=01)    -   3. No transition from 0 to 0 (reset goes high, op/on does not        change)

The third sample illustrates the pre-emptive nature of the SR latch,where enb causes op to begin to rise before reset pulls it back down.

Referring now to FIG. 9, shown is a simplified block diagram of a SerDes(serializer/deserializer) receiver path provided by an embodiment of theinvention. A SerDes is a pair of functional blocks (transmitter andreceiver) commonly used in high speed communications to compensate forlimited input/output, or to convert a high speed serial signal to aslower speed parallel circuit for digital processing. These blocksconvert data between serial data and parallel interfaces in eachdirection. The term “SerDes” generically refers to interfaces used invarious technologies and applications. The primary use of a SerDes is toprovide data transmission over a single/differential line in order toreduce the number of I/O pins and interconnects compared to a parallelinterconnection.

The receiver path depicted in FIG. 9 includes a DIN (data input) 960,CTLE (continuous time linear equalizer) 962, AGC: (automatic gaincontrol) 964, DFE (decision feedback amplifier) 966, Demux(demultiplexer) 962, and DOUT (data output) 970. DOUT 970 may, forexample have an 80 bit bus width, but other widths are possible. Thereceiver path takes a received serial input and converts it to aparallel output.

The DFE of FIG. 9 is implemented using, in part, an SR latch circuitaccording to one of the embodiments described above.

FIG. 10 is a simplified block diagram of a DFE that is used in areceiver block of a SerDes in some embodiments, and, for example, can beused as DFE 966 in the SerDes receiver of FIG. 9. This particularexample is a single tap unrolled DFE.

The DFE has an input 972 connected to tap 974. The outputs of tap 974are input to a respective slicer 978,980. The output of the two slicers978,980 is input to a combined multiplexer and latch circuit 982 havinga select input 983. The output of the combined multiplexer and latchcircuit 982 is connected to an SR latch circuit 984 which in turn isconnected to a latch circuit 986. The output of the latch circuit is therecovered data output 988, and this is also fed back as select input 983to the combined multiplexer and latch circuit 982.

In operation, weights of +H1 and −H1 are added to the input using tap974 and both potential solutions are sliced/latched with slicers978,980. The combined multiplexer and latch circuit 982 that followschooses either the +H1 or −H1 solution based on the previous solution(it decides the current bit's solution based on whether the previous bitwas a 1 or 0).

The SR latch circuit 984 of the DFE of FIG. 6 is implemented, forexample, using the SR latch circuit according to one of the embodimentsdescribed above.

It should be understood that there are a wide range of implementationoptions for a DFE making use of the SR latch circuit according to one ofthe embodiments described above, and FIG. 10 is a very specific example.

Referring now to FIG. 11 shown is a simplified block diagram of anexample system using 25G NRZ technology, that employs SerDesfunctionality, such as described with reference to FIG. 9, and includingthe use of the DFE of FIG. 10.

Shown is a first circuit board 1000 connected to a second circuit board1004, for example through a backplane or long board trace. The firstcircuit board 1002 includes fabric ASIC 1000. The second circuit board1004 includes a first PHY retimer 1006 connected to the fabric ASIC1002, and also to port ASIC or FPGA 1010 which is then connected insequence to PHY retimer 1012 and optical module 1016 which has anoptical output 1020. Interconnections between components1002,1006,1010,1012,1016 are all serial interconnections. PHY retimer1008, port ASIC or FPGA, PHY retimer 1014 and optical module 1018 havingoptical output 1022 are interconnected in a similar manner

In operation, the fabric ASIC 1002 may for example, receive data fromseveral different port devices. Using information in the data, thedevice will route data to the appropriate destination port. The PHYretimers 1006,1008,1012,1014 clean up data signals after transmissionover the backplane or long board trace, or more generally any channelthat introduces degradation. The port ASIC or FPGA reads data traversingthrough the device for the purpose of analyzing, capturing, or modifyingthe data before it is output. The optical module 1016 is an interfacebetween optical signaling and electrical signaling Typically, the fabricASIC 1002, PHY retimers 1006,1008,1012,1014, port ASIC or FPGA 1010 areeach a respective integrated circuit that is mounted to a circuit board.Each optical module 1016,1018 might include a number of integratedcircuits, with each module as a whole plugged into circuit board 1004.

The system shows a number of blocks 124 which implement SerDesfunctionality, such as that of FIGS. 9 and 10, but more generally, whichmakes use of the combined multiplexer and latch circuit functionalitydescribed previously. While the example shows such SerDes functionalitybeing employed in numerous different locations, more generally in oneembodiment, the SerDes functionality is used for at least one serial toparallel interface.

The SerDes blocks 124 convert the serial I/Os to/from parallel signalsemployed by digital circuitry within, for example, the PHY retimers,port ASIC or FPGA, Fabric ASIC or optical modules.

Another embodiment of the invention provides an integrated circuit withone or more SerDes blocks that convert serial I/Os to parallel signals;digital circuitry that operates on the parallel signals. A PHY retimer,port ASIC or FPGA or fabric ASIC are specific examples of suchintegrated circuits. It should be understood that the SerDes blocksmight be implemented as in FIG. 9, but this is not necessarily the case.More generally, whatever the implementation is, the SerDes blocks makeuse of the combined multiplexer and latch circuit functionalitydescribed above.

In an example, the present techniques can be used for regulating memorydevices (e.g., DDR 4 SDRAM devices, DDR4 register devices, DDR4controller devices), and other high speed data applications.Additionally, such techniques can be used for a variety of applicationssuch as network and/or computer storage systems, computer servers, handheld computing devices, portable computing devices, computer systems,network appliances and/or switches, routers, and gateways, and the like.

Numerous modifications and variations of the present disclosure arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the disclosuremay be practiced otherwise than as specifically described herein.

1. An SR latch circuit having an enable input and an SR latch circuitoutput comprising: a first input stage having an input for receiving aset input and having an output for producing a first component of the SRlatch circuit output, the first input stage having only one transistorthat receives the enable input, the first input stage becomingtransparent while enabled, the first input stage having a single gatedelay between the input of the first input stage and the output of thefirst input stage; a second input stage having an input for receiving areset input and having an output for producing a second component of theSR latch circuit output, the second input stage having only onetransistor that receives the enable input, the second input stagebecoming transparent while enabled, the second input stage having asingle gate delay between the input of the second input stage and theoutput of the second input stage.
 2. The latch circuit of claim 1configured to receive set and reset inputs as RZ (return to zero)signals that are timed to become valid after the enable input has causedthe input stages to become transparent.
 3. The latch circuit of claim 1further comprising: an input generating circuit that generates the setinput and the reset inputs as RZ (return to zero) signals that are timedto become valid after the enable input has caused the input stages tobecome transparent.
 4. The latch circuit of claim 3 wherein the inputgenerating circuit comprises a strong-arm slicer circuit that generatesthe set and reset inputs.
 5. The latch circuit of claim 1 furthercomprising: a storage circuit having first and second inputscross-connected with first and second outputs, the first output alsoconnected to the output of the first input stage, and the second outputalso connected to the output of the second input stage.
 6. The latchcircuit of claim 1 configured for return-to-zero signaling for set andreset inputs, and for non-return-to-zero signaling for the SR latchcircuit output.
 7. The latch circuit of claim 1 wherein the input stageshave a transparent state activated by the enable input, and the inputstages are configured so as to overpower the storage circuit while inthe transparent state.
 8. The latch circuit of claim 1 in combinationwith a preceding circuit that generates only valid combinations ofenable input and the set and reset inputs for the SR latch circuit,wherein the valid combinations comprise: for enable active low: Enb SetReset on op Valid states 0 0 1 1 0 0 1 0 0 1 1 0 0 p p

OR for enable active high: Enb Set Reset on op Valid states 1 0 1 0 1 11 0 1 0 0 1 1 p p


9. The latch circuit of claim 1 wherein the storage circuit comprisescross-coupled inverters.
 10. The latch circuit of claim 1 wherein: thefirst input stage comprises: an inverter comprising a PMOS transistorand an NMOS transistor; the one transistor connected to one of the PMOSor NMOS transistors that receives the enable input; the second inputstage comprises: an inverter comprising a PMOS transistor and an NMOStransistor; the one transistor connected to one of the PMOS or NMOStransistors that receives the enable input.
 11. An integrated circuitcomprising the latch circuit of claim
 1. 12. A SerDes (serializerdeserializer) comprising: a serializer; a deserializer comprising an SRlatch circuit having an enable input and an SR latch circuit output, theSR latch circuit comprising: a first input stage having an input forreceiving a set input and having an output for producing a firstcomponent of the SR latch circuit output, the first input stage havingonly one transistor that receives the enable input, the first inputstage becoming transparent while enabled, the first input stage having asingle gate delay between the input of the first input stage and theoutput of the first input stage; a second input stage having an inputfor receiving a reset input and having an output for producing a secondcomponent of the SR latch circuit output, the second input stage havingonly one transistor that receives the enable input, the second inputstage becoming transparent while enabled, the second input stage havinga single gate delay between the input of the second input stage and theoutput of the second input stage.
 13. The SerDes of claim 12 comprisinga DFE (decision feedback equalizer), the DFE comprising said SR latch.14. A method comprising: controlling a first input stage and a secondinput stage to be transparent with an enable input; receiving a setinput at the first input stage and receiving a reset input at the secondinput stage, the set and reset input each being a RZ (return to zero)signal that is timed to become valid after the enable input has causedthe input stages to become transparent; the first input stage generatinga first component of a complementary SR (set reset) latch circuit outputwith a single gate delay between the set input and the first componentof the complementary SR latch circuit output; the second input stagegenerating a second component of a complementary SR (set reset) latchcircuit output with a single gate delay between the set input and thesecond component of the complementary SR latch circuit output.
 15. Themethod of claim 14 further comprising: generating the set input and thereset input as RZ (return to zero) signals that are timed to becomevalid after the enable input has caused the input stages to becometransparent.
 16. The method of claim 15 wherein generating the set inputsignal and the reset input signal comprises using a strong-arm slicercircuit that generates the set and reset input signals.
 17. The methodof claim 14 further comprising: storing the SR latch circuit output. 18.The method of claim 17 wherein storing the SR latch circuit outputcomprises storing the SR latch circuit output in a storage circuithaving first and second inputs cross-connected with first and secondoutputs, the first output also connected to the output of the firstinput stage, and the second output also connected to the output of theclocked input stage.
 19. The method of claim 14 wherein generating firstand second components of the SR latch circuit output comprisesgenerating non-return-to-zero signaling for the SR latch circuit output.20. The latch circuit of claim 1 wherein the input stages have atransparent state activated by the clock, and the input stages areconfigured so as to overpower the storage circuit while in thetransparent state.
 21. (canceled)
 22. (canceled)
 23. (canceled)